Saturday, August 13, 2011

Comparing apples to apples

Sometimes it can really be hard to make meaningful comparisons between apples and apples…

But a recent review article makes a valiant effort at making such a comparison. Which is more efficient—this solar panel:

Or this one?

(The answer is coming…but make your guess now!)

It’s useful to note that this really is an apples-to-apples comparison. We’re used to thinking of life as being powered by chemical energy—you know, breaking down ATP or burning glucose, or photosynthesis making glucose. It may come as a shock that the energy underlying all these chemical processes is electrical energy—the movement of electrons from high-energy states to low energy states.

A surface view of what goes on in a photosynthesizing leaf is that energy from sunlight is used to combine carbon dioxide and water to make glucose. However, a deeper view is that this is an electrical process. Energy from sunlight is used to take a low-voltage electron, one slumming around on a molecule of water, and exalt it to an amazingly high potential. Once energized, the electron can be put onto a carbon atom*. This trick is managed by a handful of pigments, including chlorophyll, and a whole mess of protein enzymes.

The point of chlorophyll is to do the first part of photosynthesis: use light energy to give an electron a kick in the pants. Chlorophyll absorbs only certain colors of light. It loves blue and red, can use a little green and infrared, but essentially can’t use any of the other UV or other light energy that hits the earth. Different colors of light have different energies, which is why you will get a nasty burn from UV, but not red light. When chlorophyll absorbs blue light, it wastes a bunch of the energy stepping the light down in energy until it’s essentially the same energy as red light. Only then will it energize an electron, and the remaining energy is wasted as heat.

So here’s one powerful strike against photosynthesis—it only uses a fraction of the solar energy that hits the earth, and it makes inefficient use of most of that fraction. Compare that with a silicon solar cell: in principle, it can make use of any photon from UV through the visible spectrum to far infra-red. Here’s a chart (very loosely adapted from Blankenship et al) showing how many photons of different colors hit the earth:

So, lots of different colors besides the visible ROY G BIV hit the earth. In fact, since a UV photon packs more energy than a visible photon, most of the energy hitting the earth is invisible. How many of these photons—how much of the sun’s energy—can photosynthesis use?

Not so much—it can’t make much use of yellow or green, or any of the UV. How does this compare with a silicon solar cell?


The second part of photosynthesis is the synthesis: using a hot-to-trot electron to make glucose. From a casual inspection, this is amazingly efficient—nearly 100% efficient, in that every electron that gets energized finds its way to glucose, without any losses. However, this estimate has to be tempered by biological reality. Unlike solar cells, whose raison d’etre is to make voltage for our use, the point of a plant—a point shaped by billions of years of evolution—is to make another plant. So, this photosynthetic system is not just making glucose for us to burn, it’s making membranes and proteins and pigments and DNA and so on. If we measure efficiency in terms of how much of the original sunlight gets converted into energy we can use, 100% gets whittled down to slightly over 1%.

How does this compare with a silicon solar cell? The best of these converts photon energy into voltage with an efficiency of about 18%. If we want to make an apples-to-apples comparison with a leaf, then we can use our solar cell to electrolyse water and make hydrogen gas. This process has some efficiency losses, so it brings the efficiency of a silicon solar cell down to about 14%.

OK—did you guess right about which was more efficient? I sure didn’t. But, as the authors say, “the efficiency advantage clearly goes to photovoltaic systems.”

So, is silicon really greener than a leaf? Well, yes and no. photosynthesis is an evolved, not a designed system. So, many key elements of photosynthesis were jury-rigged from other parts. And, if you start with a jury-rigged system, there’s going to be severe limits on how much it can be improved. (The authors of this review article use a wonderful euphemism, “legacy biochemistry,” to describe this historical baggage that all living things carry around.) Also, there’s the pesky fact that organisms are interested in making more organisms, not helping us.

However, we now know enough about biology to do a little bio-engineering. We have reached a point where we can contemplate taking an inefficient, evolved system and subjecting it to some intelligent re-design. We can make the components more efficient, and make the system’s main purpose energy production rather than reproduction.

Chlorophyll is a good start. It’s thought to have evolved on earth at a time when other organisms had already figured out a way to use green wavelengths of light for making energy. (These organisms are still around—they give salt ponds their spectacular purple hue. If you take the spectrum of visible light and absorb all the green and a little yellow-orange, as these guys do, you are left with purple.) Therefore, chlorophyll evolved to make use of the leftovers, blue and red. UV and infrared were eschewed because they’re just too dangerous for living things to deal with. Some researchers have been tinkering with modifications to chlorophyll, and have succeeded in making it absorb new wavelengths of light.

The synthesis part of photosynthesis is also subject to tinkering: the enzyme that starts the process of making glucose is notoriously inefficient, since it first evolved on earth when there was a much higher concentration of CO2 in the atmosphere, and virtually no oxygen. In this light, it is unsurprising that this enzyme is really inefficient in the presence of oxygen. Certain plants and bacteria have developed work-arounds for protecting this enzyme from oxygen and locally increasing the concentration of CO2, but it’s easy for us to simply grow algae in a bioreactor that’s kept nearly free of oxygen, and pump in lots of CO2 from burning biomass.

There are even more radical proposals for bio-engineering photosynthesis. These are pretty far in the realm of science fiction, but who knows—they may be used to power your oft-promised flying car. The authors of this review suggest a re-engineered algae, something that could only grow in a bioreactor, a slave to our demands for energy. It would have a short life span, because its engineered chlorophylls would absorb all wavelengths of light. It would not grow especially well, because most of the energy it absorbed would be used for making fuel, rather than making more cells. And, since glucose isn’t the best fuel to power your flying car, it would energize electrons from water and use them to make hydrogen gas. Such a system may not achieve the same efficiency of a silicon cell, but the peripherals (processing, hazardous waste produced, etc) may well make it much greener.

There’s no doubt that, sometime in the next century, big oil will be replaced by something else, and that it will probably be solar. The question is, will it be big silicon or big algae?

Robert E. Blankenship et al (2011). Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement. Science 322, 805-809.

*a proton also goes along for the ride, and an electron and a proton together make a hydrogen atom—so chemically, it looks like hydrogen is being added to CO2.

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